Power MOSFETs are well known semiconductor devices. Two competing operational characteristics of power MOSFETs are the breakdown voltage and the Rdson (on resistance). Another operational characteristic of a power MOSFET that is important is its switching frequency. It is generally desirable to have a power MOSFET with high breakdown voltage, low Rdson, and a high switching frequency capability. It is also desired to have a power MOSFET having the foregoing characteristics and, in a cellular device, a high cell density in order to reduce the size of the device.
FIG. 1 shows the structure of a well known vertical conduction power MOSFET. The well-known device shown in FIG. 1 employs a silicon substrate 30 having a junction-receiving epitaxial layer 31 grown on the top surface thereof. A plurality of source regions 33 of the same conductivity as epitaxial layer 31 and substrate 30 are provided in base regions 32 of the opposite conductivity type. Invertible channels 32′ are disposed between source regions 33 and common conduction regions 35.
A thin gate oxide 34 overlies the invertible channels 32′ and the top of the common conduction regions 35. A conductive polysilicon layer 36 overlies the gate oxide layers 34 and is insulated by a low temperature oxide layer 37. The polysilicon layer 36 serves as the gate electrode structure for creating the electric field that is required to invert the invertible channels 32′ in order to electrically link the source regions 33 to the common conduction regions 35. Oxide spacers 38 are also formed on the sidewalls of the polysilicon layer 36. Oxide spacers 38 and low temperature oxide layer 37 electrically insulate the polysilicon layer 36 from the contact layer 39 which is electrically connected to the source regions 33 and serves as a contact for the same. Aluminum or some other suitable metal can be used to form the contact layer 39. It is noteworthy that in the device shown in FIG. 1 contact layer 39 extends through a depression in the source regions 33 to make contact with the base regions 32, thus shorting the source regions 33 and the base regions 32, thereby preventing the operation of the parasitic bipolar transistor in the body of the device. In a vertical conduction MOSFET such as the one shown in FIG. 1 the bottom free surface of the substrate 30 is also metallized to serve as the drain contact for the device.
The device shown in FIG. 1 is an N channel MOSFET. In this device, the base regions 32 are lightly doped with a P type dopant such as boron, while the source regions 33 are highly doped with an N type dopant such as phosphorous; the epitaxial layer 31 (or drift region) is lightly doped with an N type dopant such as phosphorous, and the substrate 30 is highly doped with an N type dopant such as phosphorous. A P channel MOSFET may be devised using the same structure as that shown in FIG. 1 but using the opposite conductivities as that shown in FIG. 1 in every region.
When a positive voltage of sufficient strength is applied to the polysilicon layer 36, an electric field is created, which field begins to deplete the invertible channels. When the channels are sufficiently depleted, the invertible channels are inverted and an N channel is formed between the source regions 33 and the common conduction regions 35. A voltage between the source regions 33 and the drain at the bottom of the device will cause a current to flow between the two.
The lightly doped region in the epitaxial layer 31 is often referred to as the drift region. In the conventional MOSFET shown by FIG. 1, this region is lightly doped in order to increase the breakdown voltage of the device. Because it is lightly doped, the drift region significantly contributes to the Rdson of the device. Hence, in conventional MOSFETs a balance must be struck between the desired breakdown voltage and the Rdson in that an improvement in one obtained by varying the concentration of dopants in the drift region adversely affects the other.
Superjunction devices are known. These device include highly doped columns or pylons usually formed under the base regions. The drift region in superjunction devices is also highly doped and has a charge that is equal to that of the highly doped pylons or columns. Due to the increase in the concentration of dopants in the drift region the Rdson of a superjunction device is less than other devices. However, the breakdown voltage of a superjunction device is not compromised due to the increase in the concentration of dopants in the drift region in that the highly doped columns or pylons cause the lateral depletion of the drift region under the reverse voltage condition thereby improving breakdown capability in the device.
A schematic of such a structure, which is often referred to as a superjunction structure is shown in FIG. 2. Referring to FIG. 2, highly doped pylons or columns 32″ are formed under the body regions 32. To take advantage of the characteristics of a superjunction charge balance must be struck between the pylons 32″ and the areas surrounding the highly doped pylons 32″. Thus, the concentration of the dopants in the drift region is increased to match those of the pylons 32″. The increase in the concentration of dopants in the drift regions reduces the Rdson of the device. However, as shown in FIG. 2, the increase in the dopant concentration does not reduce the breakdown voltage in that the pylons 32″ operate to deplete the drift region between the pylons for the length of the of the pylons thereby improving the breakdown voltage of the device. As a result, a device is obtained that has a low Rdson and a high breakdown voltage.
As explained above, to reduce the RDSON while keeping the breakdown voltage high, deep pylons or columns 32″ of one of the conductivity types are formed in the drift region of the device. The formation of pylons or columns 32″ requires many epitaxial depositions each followed by a diffusion drive. Such a process may require many masking steps which further complicate the manufacturing of the superjunction devices. Thus the manufacturing of conventionally known superjunction devices can be a time consuming and therefore expensive process.
The frequency response of a MOSFET is limited by the charging and the discharging of its input capacitance. The input capacitance of a MOSFET is the sum of the gate to drain capacitance (Cgd) and the gate to source capacitance (Cgs). As the Cgd and the Cgs become smaller the MOSFET can operate in a higher frequency range. Thus, it is desirable to have lower input capacitance in order to improve the frequency response of a MOSFET.